Introduction
Mobile phase additives are among the most important variables in LC-MS method development. The correct additive can significantly improve:
- Ionization efficiency
- Signal intensity
- Peak shape
- Retention reproducibility
- Method robustness
The optimal additive often depends on whether the method is operating in positive or negative electrospray ionization (ESI) mode. When developing LC-MS methods using Cogent™ TYPE-C™ columns, relatively low concentrations of additives are often sufficient, helping maintain sensitivity while minimizing instrument maintenance requirements.
Positive Ion Mode Recommendations
For many positive ion mode LC-MS methods, formic acid is a common starting point.
Recommended Starting Condition: 0.1% Formic Acid
Potential benefits include:
- Efficient protonation of analytes
- Improved ionization efficiency
- Strong LC-MS compatibility
- Stable chromatographic performance
- Simple mobile phase preparation
Positive ion mode is frequently used for:
- Basic compounds
- Pharmaceuticals
- Peptides
- Amines
- Many metabolites
In these applications, formic acid often provides excellent overall performance.
Negative Ion Mode Recommendations
For many negative ion mode applications, ammonium acetate is often an effective choice.
Recommended Starting Condition: 10 mM Ammonium Acetate
Potential benefits include:
- Improved response for acidic analytes
- Stable ionization conditions
- Good chromatographic reproducibility
- Compatibility with many LC-MS workflows
Negative ion mode is commonly used for:
- Organic acids
- Acidic metabolites
- Environmental contaminants
- Some pharmaceutical compounds
Why Additive Concentration Matters
Increasing mobile phase additive concentrations does not always improve method performance.
Higher concentrations may contribute to:
- Greater LC-MS source contamination
- Increased maintenance frequency
- Higher chemical background
- Ion suppression
- Reduced sensitivity
For this reason, it is generally advisable to use the lowest concentration that provides acceptable chromatographic performance.
Source Contamination Considerations
Ammonium acetate is volatile and LC-MS compatible, but it can still leave deposits within the ion source over time.
As concentration increases:
- Source contamination can increase.
- Cleaning intervals may shorten.
- Sensitivity may gradually decline.
Using moderate concentrations can help reduce these issues while maintaining chromatographic performance.
HILIC Method Development Considerations
Some traditional HILIC methods rely on relatively high concentrations of salts or buffers to achieve retention.
However, high additive concentrations can create challenges for LC-MS systems, including:
- Salt buildup
- Source contamination
- Reduced instrument uptime
- Greater maintenance requirements
Many methods developed on Cogent™ TYPE-C™ columns achieve effective retention using lower additive levels, which can improve overall LC-MS compatibility.
Solubility of Ammonium Salts
When preparing organic-rich mobile phases, additive solubility must be considered.
Important Preparation Tip
Ammonium acetate does not dissolve readily in pure acetonitrile.
For this reason, the organic mobile phase should contain at least: 5% Water This small amount of water significantly improves additive solubility and helps ensure consistent mobile phase preparation.
Using Both Formic Acid and Ammonium Acetate
In most LC-MS methods, a single additive system is sufficient. However, there are occasions when both formic acid and ammonium acetate may be used within the method.
Example
A pH-gradient strategy may require:
- Acidic conditions in one solvent
- Buffered conditions in another solvent
These approaches should be evaluated carefully during method development to ensure stability, reproducibility, and detector performance.
Recommended LC-MS Method Development Workflow
Positive Ion Mode
Start with: 0.1% Formic Acid
Evaluate:
- Signal intensity
- Peak shape
- Retention
- Signal-to-noise ratio
Negative Ion Mode
Start with: 10 mM Ammonium Acetate
Evaluate:
- Sensitivity
- Peak shape
- Baseline stability
- Source cleanliness
Keep Additive Levels Practical
Use the lowest concentration that meets analytical requirements.
Key Takeaways
- Mobile phase additive selection should be based on ionization mode.
- Positive ion mode often performs well with 0.1% formic acid.
- Negative ion mode often performs well with 10 mM ammonium acetate.
- Excessive additive concentrations can increase source contamination and maintenance requirements.
- Ammonium acetate requires water for reliable dissolution and should not be prepared in pure acetonitrile.
- Lower additive concentrations often provide better long-term LC-MS performance.
- TYPE-C™ columns frequently allow effective chromatographic retention without the need for high salt concentrations.
